There are a great many keyboard shortcuts you can use to work with Excel more efficiently. For a handy reference to all of them, some which work in other Office programs, Microsoft offers these PDF reference sheets.

You can also see the whole list of shortcuts in the article from Microsoft below (the PDF reference sheets are linked to in the article in the left sidebar). Note: Many of these shortcuts work in Office for Mac (substitute Cmd for Ctrl), but not all seem to, and if you're using the Excel Starter edition, not all of the features may be supported either.

That keyboard shortcut article also points out an interesting shortcut for the ribbon called Key Tips. Press ALT to make the key tips appear and then you can press a key to activate a button on the ribbon menu:

There really are a lot of Excel keyboard shortcuts. Excel power users: which are your favorites?